This invention relates to a titanium-clad steel which is manufactured by rolling and to a method for its manufacture. More particularly, it relates to a titanium-clad steel which includes inserts in the form of a low-carbon ferrous metal sheet and a nickel or nickel alloy sheet.
Clad metal sheets have come to be widely used due to the fact that they take advantage of the favorable properties of both the base metal and the cladding. Titanium-clad steels, for example, are useful in manufacturing desalination apparatuses. There is now particular interest in titanium-clad steels because the steel base metal compensates for the drawbacks of the titanium cladding.
At present, titanium-clad steel is manufactured either by the explosive cladding method or by the rolling method in which a low-carbon ferrous metal sheet or industrial-grade pure iron sheet (hereunder collectively referred to as a "low-carbon ferrous metal") is used as an insert.
The explosive cladding method has the drawback that there is a limitation on the size of a slab which can be assembled, and the dimensions of the resulting product are therefore small. The dimensional accuracy of the product is also poor, and the cladding tends to be lacking in uniformity. Furthermore, SR (stress relief heat treatment) produces a large decrease in the bonding strength of a clad steel produced by this method. Accordingly, the explosive cladding method is not satisfactory for general use.
In the rolling method, titanium cladding is bonded to a steel base metal by rolling at a high temperature. If the cladding is directly bonded to the steel without an insert therebetween, TiC will precipitate along the interface between the base metal and the Ti cladding. This TiC is extremely hard and brittle and inevitably produces a deterioration in the bonding strength of the clad steel, which is highly undesirable. Therefore, an insert in the form of a sheet of a low-carbon ferrous metal is usually disposed between the Ti cladding and the steel base metal in order to restrict the precipitation of TiC. However, the use of such an insert is not sufficient to solve the problem of TiC precipitation, since C has a high diffusion rate in Fe, so that C from the base metal readily passes through the insert, reaches the interface between the Ti cladding and the insert, and forms TiC. The diffusion of C through the insert is decreased as the thickness of the insert is increased, but only to a certain extent, since C in the base metal can readily pass along grain boundaries in the insert. Furthermore, the tendency for C to diffuse increases as the heating temperature is increased. Diffusion of C becomes even easier if stress relief heat treatment is performed after the thickness of the clad steel has been decreased by rolling. Thus, with the recent tendency to manufacture clad steel from thin layers which are subjected to a large amount of working, and the tendency to perform rolling at high temperatures in order to increase the reduction ratio and increase productivity, the formation of TiC in the manufacture of clad steel by rolling has become a major problem.
Japanese Patent Application Laid-Open Specification No. 53-94249 (1978) discloses a method of manufacturing titanium-clad steels in which an industrial-grade pure iron insert is employed Prior to assembly, the pure iron insert is provided on a titanium plate by means of electroplating or explosive cladding.
Japanese Patent Application Laid-Open Specification No. 56-122681 (1981) discloses a method of manufacturing titanium-clad steels in which a ultra-low carbon steel is employed as an insert or base metal. The ultra-low carbon steel comprises 0.07% or less of carbon, 0.01-0.50% of silicon, 0.30-2.00% of manganese, and at least one carbon-fixing element, such as Ti: 0.01-0.50%, Ti/C=3 or more; Nb: 0.05-0.30%, Nb/C=5 or more, and Mo: 0.05-1.00%.
Japanese Patent Application No. 58-29589 (1983) discloses a method of manufacturing titanium-clad steels in which a first insert member selected from niobium, niobium alloys, tantalum, and tantalum alloys, and a second insert member selected from copper, copper alloys, nickel, and nickel alloys are employed The first and second inserts are bonded by an explosive bonding method or diffusion bonding method and the thickness is reduced by rolling prior to assembly. The thus-combined first and second thin inserts are disposed between a titanium plate and a steel base plate, and are bonded together by an explosive bonding method, diffusion bonding method, or rolling bonding method. This process is rather complicated and costly.